This invention relates to wastewater treatment and, more particularly, to wet oxidation processes for treating wastewaters containing high concentrations of inorganic ammonium salts, such as ammonium sulfate.
Wet oxidation is used for oxidizing a compound while in solution. In a typical wet oxidation process, an oxygen-containing gas is incorporated into the wastewater influent, the influent preheated to initiate the reaction, and the preheated influent is introduced into a reaction vessel. The exothermic oxidation reaction heats the reaction mixture in the reaction vessel to an elevated temperature, therefore many wet oxidation processes typically include some sort of heat exchange scheme for recovering heat energy from the reaction vessel effluent.
In addition to employing heat exchangers for recovering heat from the reaction vessel effluent, wet oxidation processes typically include separation means for degassing the reaction vessel effluent. Some processes direct the effluent and the hot gases generated in the reaction vessel separately away from the reaction vessel (hot separation). U.S. Pat. No. 3,714,911 discloses hot separation where the effluent is passed through a heat exchanger to preheat the influent. In the oxidation processes disclosed in U.S. Pat. Nos. 3,359,200, 3,876,497 and 4,013,560 the energy from the reaction mixture is recovered in a similar manner but without first separating the gas stream. Only after the mixture has been cooled are the gases of reaction and the effluent separated (cold separation).
U.S. Pat. No. 4,234,426 discloses alternative wet oxidation processes in which steam is generated by heat recovered from the reaction vessel effluent. In one embodiment, the generated gas stream alone (hot separation) is removed from the reaction vessel and passed through a series of heat exchangers, first to generate steam and then to exchange heat between the gas stream and the influent. In the other embodiment, the entire reaction mixture, comprising combined effluent and gas streams, is passed through the heat exchangers and subsequently separation into the gas and liquid streams (cold separation).
The effluent can be further treated, either to recover certain of the components or simply to condition the effluent for discharge to the environment. Hot separation produces a more concentrated effluent because separation occurs in the upper portion of the reaction vessel where the effluent temperature is at its highest from the exothermic wet oxidation reaction. Consequently, the gas stream from the reaction vessel contains a relatively large proportion of evaporated water at this point, leaving a much smaller proportion of water in the liquid effluent stream. The condensate produced during cooling of the gas stream is treated separately, often through biological processes.
Wastewaters from the production of a variety of industrial chemicals such as acrylonitrile or caprolactam have a high concentration of ammonium compounds, as well as various other combustibles. Ammonium sulfate, present in the raw wastewaters, is also produced in the wet oxidation reaction. The effluent is typically further treated to recover the sulfur or solid ammonium sulfate (AMS). Ammonium sulfate in the effluent preferably is concentrated as much as possible before it is fed to a sulfur recovery boiler or an AMS crystallizer. A more dilute solution puts increased energy demands on the boiler or crystallizer to evaporate the water. To achieve this concentration in a hot separation process, the reaction vessel must be run at higher temperatures, in effect driving a higher portion of the liquid phase into the gas stream as vapor.
Ammonium salts decompose into ammonia and an acid when heated, the salts of weaker acids decomposing at lower temperatures than the salts of stronger acids. If the reaction vessel contains a liquid having a high ammonium content, the gas stream will have a high ammonia content and the hotter the reaction the higher the amount of ammonia in the gas stream. In addition to ammonia and water vapor, such a gas stream contains residual oxygen, carbon dioxide, low concentrations of volatile hydrocarbons and may contain nitrogen. If the ammonia and carbon dioxide concentrations in the condensate produced by cooling the gas stream are high enough, ammonium carbonate (or ammonium bicarbonate) will form as a solid. The ammonium carbonate is partially soluble in water. If the amount of water in the condensate is insufficient to dissolve all the ammonium carbonate, the undissolved or unsuspended solids will form a scale and will eventually plug the piping, particularly in the heat exchangers.
In a hot separation process of such wastes a significant fraction of the water is removed in the liquid effluent, containing the ammonium sulfate. The gas stream usually does not contain sufficient water vapor, subsequently condensed into water, for dissolving all of the ammonium carbonate solids that form in the condensate.
There is difference between wet oxidation of these wastes using air versus pure oxygen. Wet oxidation of acrylonitrile waste with air and using hot separation to get a concentrated brine solution is possible, but the vapor coolers must be operated at relatively warm temperatures to prevent formation of the ammonium carbonate or bicarbonate, as when the off gas/condensate is left a bit warmer the carbonate/bicarbonate solids either do not form or are soluble enough to not create fouling problems. If the gas/condensate flow gets too cool, the exchangers will plug.
Wet oxidation of acrylonitrile wastewater using pure oxygen and hot separation is more problematic. When using pure oxygen, the nitrogen component of air is avoided, which affects the system. The nitrogen acts to dilute the carbon dioxide and ammonia gases, and therefore reduces the carbonate/bicarbonate solids formation in the gas/condensate. The increased non-condensable gas concentration over the condensable gas concentration increases the ratio of water to CO.sub.2 /NH.sub.3 through normal evaporation. Therefore, there is more water relative to the CO.sub.2 /NH.sub.3 component of the gas phase available to dissolve any solids that may form from that component.
Consequently, it is difficult to accomplish hot separation in a continuous process with wastewaters from the production of acrylonitrile. If conditions are maintained so that the ammonium sulfate liquid stream is concentrated enough to be fed directly to conventional sulfur recovery processes the amount of ammonia relative to condensable water in the gas stream from the reaction vessel will be so high as to cause fouling problems. The condensate from a hot separation process would also contain high ammonia concentrations that would be difficult to treat before disposal.
Thus, conventional wet oxidation processes with hot separation may not be suited for treating wastewaters from the production of acrylonitrile or similar ammonium salt containing wastewaters.